Exhaust gas emitted from diesel engines contains NOx, and particulate matter, so-called particulates (PM), such as oily particulate matter, carbonaceous particulate matter and sulfuric acid mist, and moreover, hydrocarbons, carbon monoxide, etc. Various methods for purification of such diesel engine exhaust gas by removing those components therefrom have been proposed.
Conventionally, carbonaceous PM contained in diesel engine exhaust gas, i.e., unburnt carbon, is captured by a DPF, thereby being removed from the exhaust gas. As disclosed in JP-A 09-094434 and JP-A 2001-269585, a DPF is normally a honeycomb structure which is made of silicon carbide, cordierite or the like and which has many through-holes (cells) divided by partition walls along the flow direction of exhaust gas, and at both ends of which adjacent through-holes are alternately closed at one end. Exhaust gas which has flown into the DPF through the opening of one through-hole at the inlet side of the honeycomb structure passes a partition wall and is emitted through the opening at the outlet side via the adjacent through-hole. During this process, unburnt carbon is captured by the partition wall.
However, when such a DPF is used, as unburnt carbon is accumulated in the DPF, the pressure loss of the filter increases to have adverse effect on combustion of fuel in an engine, and finally the function of the DPF will be lost. As disclosed in JP-A 08-217565 and JP-A 08-312334, a method has been used in which when the pressure loss of a DPF reaches a predetermined value, rich combustion of fuel is performed to increase the exhaust gas temperature to about 700° C. and thereby unburnt carbon captured is burnt. According to such a method, a DPF can be used while it is reproduced. However, there is a problem that fuel efficiency is decreased because fuel is consumed in order to increase the temperature of DPF when it is reproduced.
In JP-A 2002-004838 and JP-A 2002-058924 proposed is a method in which such a noble metal oxidation catalyst as platinum is arranged in the preceding region of a DPF to produce NO2, thereby promoting combustion of unburnt carbon to lower the reproduction temperature of the filter, or a noble metal oxidation catalyst is supported on a DPF, thereby to similarly lower the reproduction temperature of the filter. It is believed that, in such a method, NOx is also purified to some extent by hydrocarbons and carbon monoxide contained in exhaust gas in the presence of that catalyst.
It is known that by use of a DPF and a noble metal oxidation catalyst in combination, NOx in exhaust gas, especially NO2, promotes combustion of unburnt carbon. On the other hand, however, as disclosed in JP-A 01-318715 and APPLIED CATALYSIS: B50 (2004), 185, it has already been known also that NO2 is only reduced to NO after contributing to the oxidation reaction of unburnt carbon and that the oxidation reaction of unburnt carbon has no contribution to reduction of NOx to nitrogen, namely, reduction in the amount of NOx. Moreover, in the oxidation reaction of unburnt carbon on a noble metal oxidation catalyst such as platinum, oxidation of unburnt carbon proceeds rapidly and unburnt carbon is exhausted promptly. Therefore, even if NOx is purified, the purification reaction will stop instantly and, therefore, the amount of NOx purified by unburnt carbon is very small.
On the other hand, in JP-A 09-094434 and WO02/096827, there is proposed a method in which NOx and unburnt carbon can be removed simultaneously by using a DPF which supports a NOx occlusion reduction catalyst thereon, thereby occluding NOx during lean combustion of fuel and purifying NOx and unburnt carbon during rich combustion of fuel. In such a method, the degree of rich combustion of fuel can be reduced to some extent because unburnt carbon is used for a part of NOx reduction. It, however, does not fundamentally improve the deterioration of fuel efficiency because it is still necessary to perform rich combustion of fuel.
A catalyst is also proposed for removing NOx and unburnt carbon simultaneously as mentioned above in JP-A 2006-289175. The catalyst comprises a solid superacid and platinum or the like having high oxidizing ability supported thereon. It, however, seems impossible to purify NOx over a wide temperature range because combustion of carbon, carbon monoxide and hydrocarbons proceeds rapidly and, as a result, a reducing agent disappears rapidly.
Moreover, a composite oxide having perovskite structure or spinel structure containing a metal having a high perfectly oxidizing ability and a low electronegativity is proposed as a catalyst for removing NOx and unburnt carbon simultaneously in JP-A 2003-239722 and APPLIED CATALYSIS: B 34 (2004), 29. It, however, seems impossible to purify NOx over a wide temperature range because this catalyst also has a high perfectly oxidizing ability and, therefore, combustion of carbon, carbon monoxide and hydrocarbons proceeds rapidly and, as a result, a reducing agent disappears rapidly. In addition, this catalyst has a problem that the oxidizing ability and reducing ability thereof will be lost in the presence of sulfur oxides because the catalyst contains a metal having a low electronegativity.
Under these circumstances, a catalyst and a method are awaited which can remove NOx and unburnt carbon simultaneously under normal lean driving conditions even in the presence of sulfur oxides without needing rich combustion of fuel to reproduce a DPF, which has been performed. Moreover, in order that such a catalyst and method may be applied to diesel engines, it is strongly desired that a reaction to remove NOx and unburnt carbon simultaneously proceeds over a wide temperature range. The reason for this is that since the exhaust gas temperature of a diesel engine driven under various conditions varies greatly, in order to cause a reaction of simultaneous removal of NOx and unburnt carbon to proceed and, at the same time, to reproduce a DPF, the reaction must proceed over a wide temperature range.
It is, therefore, an object of the invention to provide a catalyst and a method not only for catalytically reducing NOx in diesel engine exhaust gas using unburnt carbon contained therein as a reducing agent in a wide temperature range even in the presence of sulfur oxides; at the same time for catalytically removing the unburnt carbon.